Fillers typically are added to cellulose fibers during alkaline paper making to improve brightness, opacity, show through, and feel of the formed paper. Fillers used in alkaline paper making processes typically include fine particles of inorganic materials such as clay, titanium dioxide, and calcium carbonate such as precipitated calcium carbonate ("PCC").
As much filler as possible is employed in alkaline paper because the filler not only provides improved optical properties, but also because the filler is less expensive than the pulp fiber. Additionally, increasing the amount of filler in alkaline paper making reduces energy consumption. However, increased amounts of filler may detrimentally affect the strength of the paper, both in terms of tensile strength as well as internal bond strength.
It is known to treat filler materials used in paper making to improve the strength of the paper. For example, GB 1,505,641 discloses treating particles of inorganic materials with an anionic latex of a synthetic polymeric resin. Similarly, U.S. Pat. No. 4,445,970 discloses a method of manufacturing paper containing mineral filler and anionic latex.
GB 2,016,498 discloses flocculating paper making fillers with a composition comprising starch, an organic polyelectrolyte, and an agent for moderating the degree of flocculation and viscosity of the dispersion. The resulting pre-flocculated filler is disclosed to provide improved tensile strength in paper.
U.S. Pat. No. 4,710,270 discloses a mineral filler covered with a dispersion of cationic starch and carboxymethylcellulose or alginate, resulting in improved strength and retention.
U.S. Pat. No. 4,799,964 discloses a preflocculated filler formed by continuously contacting a flow of an aqueous pigment slurry with a flocculating agent, followed by imparting sufficient shear force to control the particle size of the preflocculated filler. The resulting preflocculated filler is disclosed to exhibit improved retention.
U.S. Pat. No. 4,181,567 discloses contacting a mineral filler with a relatively high molecular weight homopolymer or copolymer of acrylamide to form an agglomerate which is disclosed to improve mechanical properties of the paper.
EP 399,775 discloses a film forming composition for paper comprised of a gum, a water-soluble alginate, starch, and a polyvinyl alcohol or carboxymethylcellulose. The resulting composition is disclosed as providing improved sizing and porosity control.
U.S. Pat. No. 4,420,341 discloses a stabilized surface modified filler consisting of an organic acid reactive filler, an unsaturated organic acid, an antioxidant, and a hydrocarbon liquid agent. The surface modified filler is disclosed to improve various physical properties when used in rubber and plastic compounding.
U.S. Pat. No. 2,322,185 discloses using karaya gum or alkali treated karaya gum to flocculate mineral fillers for improving retention and recovery of mineral fillers.
U.S. Pat. No. 2,949,397 discloses coating filler particles with organic colloid material derived from plant seeds or corns such as plant mucilage. An example is substituted mannans which are complex polysaccharides. The disclosed advantage is improved retention.
U.S. Pat. No. 4,755,259 discloses a process for papermaking in which a binder comprising colloidal silicic acid and either amphoteric or cationic guar gum is added to the papermaking furnish prior to sheet formation. The binder addition results in improved retention and strength.
U.S. Pat. No. 5,104,487 describes a papermaking process using a cationic starch in combination with a naturally anionic polysaccharide gum. The resulting paper or paperboard exhibits improved strength.
None of the above patents address the problem of decreased tensile strength and internal bond strength in alkaline paper products as the percent of filler in the paper is increased. None of these references, moreover, show reacting anionic or anionically modified polysaccharides with a finely divided inorganic material to obtain a filler material which provides improved strength in paper.
A need therefore exists for filler material that can be employed in alkaline paper products at increased levels but which avoids the disadvantages of the prior art.